This invention pertains to stamping foils and methods for producing electrically conductive paths on nonconductive or poorly conductive substrates.
Electrical and electronic circuits are made often of printed wiring or foil on insulating boards. Their usual manufacturing processes expend materials and labor and require complicated equipment. On substrates coated with copper (subtractive technique or metallizing technique) or with a bonding agent containing a sensitizer (additive or semi-additive technique) the conductive patterns are brought out by screen printing or photoprinting, and the printed circuit boards are etched, depending on the process used, after an initial strike of copper by chemical displacement or plating followed by electrodeposition of copper or tin. Between these main procedural steps are other necessary steps, such as cleaning, removal of the mask, drying, checking, etc. Whereas all other parts of switching or measuring instruments, such as housings, mechanically movable parts, mechanical supports or connections, can be produced by methods which are capable of a high output per unit time (e.g., pressure diecasting, stamping, drawing, etc.), the production of printed circuit boards by wet chemical means consumes a disproportionately high amount of time and work.
It is an object of this invention to provide a simple and economical method of producing printed circuit boards which is capable of a high output per unit time.
Various-dry processes have been proposed to lay down printed circuits from a material in a roll directly upon the substrates. These processes may be classed into the following three categories.
The first category includes punching the printed circuit pattern out of a self-supporting conductive foil which is laid down on a substrate, and simultaneously bonding the pattern to the substrate. The common disadvantages of this procedure are: a relatively costly punching and blanking tool, the difficult or troublesome removal of the residual foil (i.e., the portion not used as a conductive circuit) from the substrate, especially where the conductive circuits have the form of almost closed loops, and difficulties in producing very narrow conductive patterns with tight spacings in between them.
The second category comprises processes in which conductive patterns prepared by previously mentioned wet chemical methods are placed on a carrier tape and are transferred therefrom onto the substrates. The disadvantage here is that the preparation of the conductive patterns requires the aforementioned expensive wet chemical steps. Furthermore, the conductive patterns may get distorted during their transfer to and from the carrier tape.
In the third category, the conductive patterns are cut out of a copper foil placed on a carrier tape, and are thereafter transferred from the tape onto the substrates. Once the copper foil is affixed to a substrate, the carrier tape is pulled off. The problem here is with the troublesome and not very precise cutting step.
It is a second object of this invention to provide a dry process for producing printed circuit boards which is capable of producing fine and intricate circuit patterns to the required precision and which is free from the other above-cited shortcomings of the existing dry processes.
Besides the above-mentioned processes for producing printed circuits, there is also the hot-stamping process which is capable of forming thin films of conductive materials in nearly any desired pattern but which could not be used heretofore for the production of printed circuits. The layers which can be stamped out in various patterns from conventional hot-stamping foils have maximum thicknesses of about 100 millimicrons, which is too thin for printed circuits by about two orders of magnitude. The small thickness of the conductive layer (e.g. of copper) results not only in an irreproducible value of the electrical resistance due to variations in quality and to diffusion of bonding agents into the layer. A further disadvantage of metallic layers deposited from conventional hot-stamping foils is that they cannot be satisfactorily tinned or soldered due partly to their thinness and partly to a poorly solderable surface. Enhancement of the stamped conductive patterns by electroplating would not only nullify the advantage of a quick uncomplicated production method, but would also involve electroplating of pieces with poor contacts, etc. The plating bath would also get contaminated with bonding material which to some extent would diffuse into the very thin conductive layer, and the plating uniformity would therefore by unsatisfactory due to the irreproducible and non-uniform electrical resistance distribution. Finally there would arise bonding problems and durability problems.
The hot stamping process would otherwise offer considerable advantages, were it suitable for conductive patterns of the required thickness. In distinction from the afore-cited dry processes, the bonding of the conductive patterns onto the substrate and the removal of the residual foil could be accomplished in a single operation, and the required stamping would be relatively inexpensive (cheaper than a punching press). In contrast to that class of stamping processes which might as well be designated as "punching" or "blanking," hot-stamping and cold-stamping as used in this invention does not involve a blanking tool. Therefore, the residual foil can easily be removed by a carrier type as will be described below. The hot-stamping process permits, in principle, to print patterns on either flat or curved parts. It is thus possible to print numbers on the plastic counting wheels of counter registers. The main advantages of the dry processes over the aforementioned wet chemical techniques stand out with hot-stamping. Besides overcoming the already mentioned problems encountered with existing processes for printed circuit production, hot-stamping would also eliminate another disadvantage, namely the need to use separate plates for wiring and switching, even though there is frequently enough room for both on certain existing insulating parts, e.g., on housing or receptacle parts. Furthermore, a circuit plate in an instrument could frequently be made smaller, more compact, and more flexible, were it possible to resort to more complexly curved substrate plates than are compatible with present techniques.
It is therefore a further object of this invention to provide stamping foils and methods with which conductive paths can be produced in the form of printed circuits or printed wiring on insulating or poorly conductive substrates. The latter may be already mechanically functioning parts (e.g., of a housing, support, and the like) in instruments (e.g., switching, regulating or measuring instruments) or may be specially made in a compact structurally appropriate form.